1. Field of the Invention
The present invention relates to flux compositions and more particularly to basic flux compositions which are used to cover liquid steel in a transfer vessel such as a continuous casting machine tundish.
2. Description of the Prior Art
In the continuous casting of steel, a tundish is used as an intermediate vessel between a ladle and a mold to provide a reservoir of liquid metal, and to distribute the liquid steel to the mold. It has been recognized that flux compositions can be used to cover the liquid metal in the tundish during the casting operation in order to remove non-metallic inclusions such as silicates and aluminates from the liquid metal, to provide a barrier to reoxidation and to provide thermal insulation.
Heretofore, in order for tundish fluxes to provide all three of the aforementioned functions, different types of fluxes needed to be added to the surface of the liquid steel. A first type of flux was added to the surface of the liquid steel, whereby, it melted to form a basic liquid layer that served to purify the liquid metal by, for example, removing inclusions from the liquid steel. A second type of flux was added to the surface of the liquid steel to form a non-liquid, powdery or granular layer to thermally insulate the liquid steel. In some instances, the second type of flux was added directly to the surface of the first liquid flux layer to form the non-liquid layer above the liquid layer.
Most tundish flux compounds in use today are in the form of powder or larger grains and contain large quantities of silica and carbon. In these compounds, silica content can range from 50%-90% with carbon contents generally in the range of 4%-10%. The presence of free carbon in these materials retards the melting and/or formation of the liquid flux and improves the flow and spreading capability of the material. These materials are insulating, but can lead to contamination of the steel with silicon and silicates, and carbon pickup by the liquid steel.
Other prior art tundish flux compounds which contain lower levels of silica and substitute quantities of magnesia or lime have significant problems with fluidity forming a hard, impenetrable surface layer. This is overcome through the addition of calcium fluoride (spar). These additions, however, are dangerous to the environment as a result of fluorine emission, cause loss of insulation, and substantial refractory erosion.
Prior art tundish flux compounds also do not address the problems associated with high levels of impurities such as iron oxide and manganese oxide in these materials. The presence of iron oxide and manganese oxide leads to oxygen contamination of the steel and, thereby, dirties the steel by creating inclusions.
U.S. Pat. No. 5,028,257 teaches a metallurgical flux composition, however, that composition does not teach a two component system and teaches very high lime/silica and lime+magnesia/silica ratios. The significance of these high base/acid ratios is that the composition will suffer problems with lack of fluidity and, therefore, be unable to form and/or maintain a liquid flux layer for absorption of non-metallic inclusions from the liquid steel. Furthermore, this reference is not taught to contain highly fluxing oxides such as soda, lithia or potassia. Nor is there any mention of limitations on the amount of impurities such as iron oxide or manganese oxide or any teaching of gas evolution when the composition heats up during liquid metal contact to cause the rapid formation of a uniform covering layer to provide insulation and a barrier to reoxidation of the liquid metal.